The OFDM overlay schematically illustrated in Fig. REF a) can be described as follows. A block of \(L\times K\)  complex symbols, typically referred to as a frame, is defined in the DD domain. We consider the DD and the Zak domain equivalent, and thus, we use the notation introduced in Sec.  and denote the frame by \(\) . A frame represents the fundamental rectangle of a \(KL\) -periodic sequence. The frame \(\)  in the DD domain is then mapped to a frame \(a\)  in the TF domain. The individual symbols \(a_{m,l}\)  are obtained via the SFFT, which is defined as [1]}
\(a_{m,l}\triangleq \frac{1}{\sqrt{KL}} \sum _{n=0}^{L-1} \sum _{k=0}^{K-1} [n,k]e^{j2\pi \left(\frac{m}{K}k-\frac{l}{L}n\right)},\) 
